1. Field of the Invention
The invention relates to electronically steered arrays for broadband applications, and more particularly to means for actively matching such arrays as the steering angle or operating frequencies are varied.
2. Description of the Prior Art
Electronically steered arrays or phased arrays as they are also known, are arrays of antenna elements disposed at regular intervals over a surface. Each such array may either be used singly or associated with other arrays. When used with other arrays, a frequent objective is to provide complete azimuthal coverage around the antenna site. In such applications, the steering of each array should be adequate to cover a substantial fraction of the 360.degree. required for complete coverage. Conventionally the steering is plus or minus 60.degree. or plus or minus 45.degree., which allows for complete azimuthal coverage with three or four arrays.
As is well known, the impedance of each antenna element varies in the complex plane with the steering angle, varying more rapidly at larger steering angles. This effect, which increases the VSWR (Voltage Standing Wave Radio) in the antenna coupling paths, degrades the antenna performance very sinificantly at the higher steering angles, demanding smaller steering angles and more rather than fewer arrays to achieve coverage over a given solid angle.
Impedance matching is not a simple task, since the antenna impedance varies both with steering angle orientation and frequency. The impedance varies in a different manner for steering which occurs parallel from that which occurs perpendicular to the E field of the element. The antenna impedance also varies, usually by rotation in the complex plane, as the freuency of operation is shifted.
In practice,antennas in electronically steered arrays are often required to operate at substantial steering angles both perpendicular and parallel to the E fields of the antenna elements, and often at other than the resonant frequency of the antenna element. Under such conditions of steering and operating frequency, the antenna no longer matches the impedance of the driving circuit in transmission and the receiving circuit isno longer matched to the antenna in reception.
The problem of antenna matching is complicated when the antenna is electronically steered in mutually perpendicular directions, since the complex impedance of the antenna, if mapped in the complex plane, as on a Smith chart, pursues separate, angularly displaced paths. sould the frequency vary as well,the prior paths tend to be rotated. Thus areas of the Smith chart completely encircling the origin represent potential impedance states of the antenna elements. Accordingly, it is desirable to provide a matching network which can provide an acceptable impedance match to any region within a VSWR circle of a given radius in the complex plane.
Since tuning and steering are rapidly adjusted in an electronically steered array, and subject to digital electronic control, it is desirable that thematching be "agile" and adjustable at the same rate as the tuning and steering and subject to digital electronic control.
Finally, since the matching networks must be used with each antenna element of a phased array, large numbers are required. This forces an economy in the design. In addition, in reducing the VSWR's and the usually smaller dissipative losses, it is essential that the matching networks not compound the problem by introducing significant dissipative losses of their own or requiring significant amounts of dc power.